Formation fracturing method

ABSTRACT

A method for producing multiple fractures in earth formations in which the lines of least principal stress deviate substantially from vertical is described. A generally vertical borehole is drilled into the formation, the formation is hydraulically fractured from the vertical borehole, the plane in which the fracture lies is determined, a slanted borehole is drilled out from the vertical borehole in a direction such that the azimuth of slanted borehole is generally perpendicular to the plane of the fracture and then the formation adjacent the slanted borehole is hydraulically fractured at a plurality of positions along the length of the slanted borehole. When the direction of lines of least principal stress is known for the formation, the steps of hydraulically fracturing from the vertical borehole and determining the plane in which the fracture lies may be omitted and the slanted borehole is drilled in the azimuth parallel to the known lines of least principal stress.

.LDO'DUUQ United States Patent 1191 Raleigh 1 Apr. 22, 1975 FORMATIONFRACTURING METHOD Wilson, Drain-I-lole Fracturing in Stimulation [76]Inventor: Cecil B. Raleigh, 762 La Para. PflIO Wells world 145 and Alto,Calif. 94306 Primary E.\'aminerEmest R. Purser [22] Flled' 1973Assistant Examiner-Jack E. Ebel [211 App]. N0.: 346,862

[57] ABSTRACT [52] [1.8. CI. 165/1; 165/45; 166/271; A method forproducing multiple fractures in earth 166/308 formations in which thelines of least principal stress [51] Int. Cl. FZSd 21/00 deviatesubstantially f Vertical is described. A [58] Field of Search 60/26;165/1. 45; 166/50. rally vertical borehole is drilled into theformation,

166/271, 269, 303 the formation is hydraulically fractured from theverti- I cal borehole, the plane in which the fracture lies is de- [56]References cued termined, a slanted borehole is drilled out from theUNITED STATES PATENTS vertical borehole in a direction such that theazimuth 2369.497 11/1956 Rcistle 166/308 of Slanted borehole isgenerally perpendicular to the 3.020954 2/1962 Graham ct al. 166/308plane of the fracture and then the formation adjacent 3,285.335 11/1966Rcistlc 166/308 the slanted borehole is hydraulically fractured at 213.31334 /196 Huitt c t a1 166/308 plurality of positions along thelength of the slanted 3391-739 7/1968 vcnghmms 166/308 borehole. Whenthe direction oflines of least principal 2 Doggcn et stress is known forthe formation, the steps of hydrauf 'fii 26 lically fracturing from thevertical borehole and determining the plane in which the fracture liesmay be 3,593,791 7/I97l Parker 166/308 3,679,264 7 1972 Van Huiscn166/269 ommed and the Slanted borehole dnlled the OTHER PUBLICATIONSClark et al., Vertical Hydraulic Fracturing," Oil And Gas Journal, Aug.9 1954, pp. 104, 107 and 108.

muth parallel to the known lines of least principal stress.

5 Claims, 2 Drawing Figures FORMATION FRACTURING METHOD BACKGROUND OFTHE INVENTION This invention relates to a method for producing aplurality of non-coplanar and approximately parallel fractures in anearth formation adjacent a borehole to facilitate the recovery of aresource contained in the formation.

Nearly forty years ago it was found that oil bearing formations could befractured by introducing low pene tration fluids into a borehole underhydraulic pressure sufficiently high to cause propagation of a fracturefrom the borehole and that fracturing was generally followed by anincrease in oil production from the borehole. Studies of the hydraulicfracturing process have shown that the fractures are generally planarand oriented perpendicular to the direction of least principa stress(commonly designated S in the rock.

In a great many formations or formation zones. the direction of leastprincipal stress is approximately horizontal and the planes ofhydraulically produced fractures are generally vertical andperpendicular to the direction of least principal stress in the rock. Insuch formations or formation zones hydraulic pressure exerted in anapproximately vertical well produces just a single approximatelyvertical planar fracture. While hydraulically induced fractures maypropagate away from boreholes distances of 100 meters or so. the benefitobtained from a single fracture is limited and it would be desirable toproduce a plurality of generally parallel fractures in the formation andso obtain greatly increased benefits from the fracturing technique.

BRIEF DESCRIPTION OF THE INVENTION volves first drilling anapproximately vertical boreholein the formation to a depth eitherpenetrating, or very close to the horizon of. the resource bearing zoneof the formation. A slanted borehole is then drilled into the formationfrom the lower part of the vertical borehole. Optimally, the azimuth(i.e., the compass direction of the horizontal line defined by theintersection of the vertical plane containing the line of the slantedhole with the surface) of the slanted borehole would be the same as thedirection of lines of least principal stress in the rock. To obtain thebenefits of this invention. however. a slanted borehole need not be inprecisely the same direction as the direction of the lines of leastprincipal stress in the formation but may be in a direction such thatthe angle between the line of the slanted borehole and the lines ofleast principal stress in the formation is not more than 60. After theslanted borehole has been drilled to the desired depth, the slanted holeis completed and cased. Hydraulic fractures are then propagated from theslanted borehole and the fractures may be propped with sand inconventional manner to hold them open. Thefractures are produced inconventional manner by packing off sections of the slanted borehole sothat the injected fluid does not enter into existing perforations.

DETAILED DESCRIPTION OF THE INVENTION l5 ond slanted and vertical borehole. the second slanted bore hole being drilled so that it communicateswith at least the major proportion of the fractures propagated from thefirst slanted borehole.

Referring now to FIG. 1 of the appended drawings.

vertical borehole l is drilled into the formation so that it bottomseither in the resource containing zone of a formation or close to thehorizon of the resource containing zone. Slanted borehoe 2 is thendrilled into the resource containing zone C the formation from the pointin the lower part of the.vertical borehole. The formations chosen forthe application of the method are those in which the direction of thelines of least principal stress are generally horizontal and in whichthe planes of hydraulic fractures, which are known to be generallyperpendicular to the lines of least principal stress. are generallyvertical.

The direction of the slanted borehole is such that the angle between theline of the slanted borehole and the lines of least principal stress isnot greater than preferably not more than 45 and optimally less than 30.Many formations have been drilled and studied to the point that thedirection of least principal stress is already known and existinginformation permits the opof least principal stress is not known andmust be deter- ;Qgglinined before the slanted borehole is drilled. Thedife' ction of least principal stress of such formations may bedetermined by hydraulically fracturing the formation adjacent the lowerpart of the vertical borehole and then determining the plane in whichthe fracture lies. Methods for determining the position of the fractureplane are well-known and readily available as by the use of impressionpackers or by injecting radioactive tracers into the fracture and thendetermining the position of the plane from the pattern of the signalsemitted by the radioactive material in the fracture. After the positionof the fracture plane has been determined, the direction of the lines ofleast principal stress become known since they are perpendicular to thefracture plane.

When the position of the fracture plane is so determined. the directionof the slanted hole may be described either in terms of the angle itmakes with the lines of least principal stress or in terms of the angleof incidence which the borehole makes with the fracture plane. the angleof incidence being the angle between the line of the slanted boreholeand the perpendicular to the fracture plane at the point of intersectionof the lines of least principal stress in less than 60 or as a directionsuch that the angle of incidence. i.e.. the angle between the boreholeand the perpendicular to the fracture plane. is less than 60.

After slanted borehole 2 has been drilled to the desired depth. it iscompleted and cased. Fractures 3 and 9 inclusive are then made byperforating the casing at the shallowest practical depth. adjacent thearea of fracture 3. and hydraulically fracturing the formation tomaximum distance of fracture propagation consistant with economicconsiderations. This first fracture is propped with sand for gas and oilproduction and also may be propped for geo-thermal power production. ifdesired. At a few meters greater depth. the casing is perforated again.packed off and the fracturing operation is repeated. The fracturing hereand in all successive positions down the hole will be conducted inpacked off sections ofthe bore hole so that the injected fluid does notenter existing perforations.

The fracturing method above described can be used to increase therecovery of gas or oil from low permeability formations. It can also beused for in-situ. recovcry of oil from oil shale. or solution mining orextracting goo-thermal energy from subterranean formations.

There are large areas on the earth where hot. impermeable rock isaccessible to drilling. Geo-thermal energy may be extracted by drillingand fracturing the hot formation as described in connection with FIG. 1of the drawings. then pumping water down the hole into contact with thefractured surfaces. permitting the water to reside in the formation fortime sufficient to heat it and then permitting the water to reissue fromthe same hole as steam or super-heated water.

FIG. 2 of the appended drawings illustrates a modification of theinvention which is particularly well adapted to either recovery ofgeo-thermal energy or solution mining. Vertical well 10 is drilled toappropriate depth and if the direction of the lines of least principalstress in the formation are not known. as they probably will not be. theformation adjacent the lower part of vertical well 10 is hydraulicallyfractured and the plane of fracture is determined. Slant borehole 11 isthen drilled in a direction such that its angle of incidence. i.e.. theangle between the borehole and the perpendicular to the plane offracture. is less than 60. When the slant well has been drilled toappropriate depth. a plurality of non-coplanar approximately parallelfractures are produced adjacent the slanted well illustrated byfractures 12 through 18 inclusive ori the drawing. A secondapproximately vertical borehole 20 is then drilled and a second slantedborehole is drilled from a point near the bottom of the second verticalborehole and in a direction approximately parallel to that of the firstslanted borehole. The second slanted borehole is so spaced from thefirst slanted borehole that it intersects most of the fractures whichwere produced from the first slanted borehole. It is important that thesecond slanted borehole intersect the major portion of the fracturesextending out from the first slanted borehole. In areas where thecharacteristics 'of the formations are well known. it is possible tospace the separate second slanted borehole apart from the first slantedborehole by a distance such that the desired fracture penetration by thesecond borehole will be achieved. In areas where the characteristics ofthe formation are not well known the second slanted borehole can bedrilled to a depth where some of the fractures should have beenintersected. A radioactive tracer can then be injected into the firstslanted borehole and into the fractures propagated from it and thesecond slanted borehole can be logged for the presence of radioactivematerial. Alternatively. fluid can be injected into the first verticalborehole and first slanted borehole at high pressure (in excess of theparting pressure) into the fractures extending from the first slantedborehole and. if intersection of the fractures has been accomplished. apressure increase will be produced quickly in the second slantedborehole. Failure to intersect the fractures can be corrected by thenaltering the inclination of the second slanted borehole or byre-drilling at a deeper level. The second slanted borehole. whencompleted. may be cased and perforated at the points of intersectionwith the fractures. After the drilling and fracturing has beencompleted. water is pumped down borehole 10 into slanted borehole 11 andforced into the several fractures propagated from borehole 11. The watercontacts the hot subterranean rock along the fracture surfaces producingsteam and the steam flows into the second slanted borehole l9 and isrecovered at the surface through vertical borehole 20.

In some situations it may be desired to avoid the expense of drillingtwo vertical and two slanted boreholes to recover geothermal energy. Inthis event the pattern shown in FIG. 1 may be used. Water is injectedinto the formation and held under pressure for a time sufficient toproduce superheated water and steam or water above its criticaltemperature. The pressure is then released to permit flow of superheatedwater and steam to the surface. Successive cycles ofinjection andrecovery are then used to remove geothermal energy.

I claim:

I. A method of opening an earth formation in which the lines of leastprincipal stress deviate substantially from the vertical to facilitaterecovery of a resource held in the formation which comprises the stepsof:

a. drilling an approximately vertical borehole into th formation;

b. drilling a slanted borehole extending from the vertical borehole intothe formation in a direction such that the angle between the slantedborehole and the lines of least principal stress in the formation is notmore than 60; and

c. hydraulically fracturing the formation adjacent the slanted boreholeat a plurality of positions along the length of the slanted borehole.

2. The method of opening an earth formation in which the lines of leastprincipal stress deviate substantially from the vertical to permitrecovery of a resource held in the formation which comprises the stepsof:

a. drilling an approximately vertical borehole in the earth formation;

b. hydrofracturing the formation adjacent the lower portion of theborehole;

c. determining the plane in which the fracture lies;

(1. drilling a slanted borehole extending from the vertical boreholeinto the formation in a direction such that the angle between theslanted borehole and the perpendicular to any plane parallel to theplane determined in step (c) which is intersected by the borehole isless than 60; and

e. hydrofracturing the formation adjacent the slanted borehole at aplurality of positions along the length of the slanted borehole.

3. The method defined in claim 2 wherein the angle between the boreholeand the perpendicular to the plane is less than 4. The method defined inclaim 1 characterized by a further step of drilling a second verticalborehole adjacent the first borehole and a second slanted boreholeextending from the second vertical borehole into the formation generallyparallel to the first slanted borehole and spaced from the first slantedborehole by a distance such that the second slanted borehole intersectsat least a major proportion of the formation fractures produced in step(c) of claim 1.

5. The method of recovering heat from a subterranean geothermal zonewhich comprises the steps of:

a. drilling an approximately vertical borehole penetrating saidgeothermal zone:

b. hydrofracturing the formation adjacent the lower portion of theborehole;

c. determining the plane in which the fracture lies;

d..drilling a slanted borehole extending from the vertical borehole intothe geothermal zone in a direction such that the angle between theslanted borehole and the perpendicular to any plane parallel to theplane determined in step (c) which is intersected by the borehole isless than 60;

e. h ydrofracturing the formation adjacent the slanted borehole at aplurality of positions along the length of the slanted borehole;

f. injecting water into the borehole to contact the geothermal zoneunder pressure.

g. holding the injected water under pressure until it is heated toelevated temperature and.

h. releasing the pressure to permit flow of heated

1. A method of opening an earth formation in which the lines of leastprincipal stress deviate substantially from the vertical to facilitaterecovery of a resource held in the formation which comprises the stepsof: a. drilling an approximately vertical borehole into the formation;b. drilling a slanted borehole extending from the vertical borehole intothe formation in a direction such that the angle between the slantedborehole and the lines of least principal stress in the formation is notmore than 60*; and c. hydraulically fracturing the formation adjacentthe slanted borehole at a plurality of positions along the length of theslanted borehole.
 2. The method of opening an earth formation in whichthe lines of least principal stress deviate substantially from thevertical to permit recovery of a resource held in the formation whichcomprises the steps of: a. drilling an approximately vertical boreholein the earth formation; b. hydrofracturing the formation adjacent thelower portion of the borehole; c. determining the plane in which thefracture lies; d. drilling a slanted borehole extending from thevertical borehole into the formation in a direction such that the anglebetween the slanted borehole and the perpendicular to any plane parallelto the plane determined in step (c) which is intersected by the boreholeis less than 60*; and e. hydrofracturing the formation adjacent theslanted borehole at a plurality of positions along the length of theslanted borehole.
 3. The method defined in claim 2 wherein the anglebetween the borehole and the perpendicular to the plane is less than30*.
 4. The method defined in claim 1 characterized by a further step ofdrilling a second vertical borehole adjacent the first borehole and asecond slanted borehole extending from the second vertical borehole intothe formation generally parallel to the first sLanted borehole andspaced from the first slanted borehole by a distance such that thesecond slanted borehole intersects at least a major proportion of theformation fractures produced in step (c) of claim 1.